In pressurized water nuclear reactors, the reactor core cooling circuit, or primary circuit, comprises at least two cooling loops, each containing a steam generator and a primary pump.
The primary pumps are composed of a volute, inside which a bladed wheel turns which is rigidly fixed to the bottom end of a drive shaft connected to a motor.
Leaktightness along the drive shaft is achieved by a system of seals disposed in an annular space between the shaft and a casing surrounding this shaft from the point where it passes out of the volute, as far as the drive motor.
The sealing device for the primary pump drive shafts is generally composed of three seals comprising a fixed portion fastened to the casing and a movable portion fastened to the shaft.
The facing surfaces of these sealing elements are either in rubbing contact, in which case the seal is of the mechanical type, or separated by a layer of fluid circulating between the surfaces of the seal, in which case the seal is of the hydrostatic type.
Seals of the mechanical type are generally used for ensuring leaktightness between two zones in which the pressures are not two different from one another, while hydrostatic seals can be used when there is a very great difference in pressure between the two sides of the seal.
In the case of the primary pumps, the water circulated by the pump is at a very high pressure, of the order of 150 bars. The seal disposed in the most upstream position on the drive shaft, i.e., the nearest to the internal part of the pump, is therefore a hydrostatic seal which permits a substantial pressure drop between its upstream side and its downstream side, whereas the seals disposed downstream are generally seals of the mechanical type.
A circuit supplying cold water under high pressure makes it possible to introduce into the annular space delimited by the casing, upstream of the hydrostatic seal, water one part of which is delivered towards the pump volute and another part of which supplies the leakage current of the hydrostatic seal. After passing through the hydrostatic seal, this water is also used for cooling the mechanical seals.
A hydrostatic seal of the kind used as the upstream seal in primary pumps has, for example been described in French Patents Nos. 1,435,568 and 2,049,690.
If a hydrostatic seal of this kind is to function correctly, i.e., without bringing into contact the elements disposed opposite one another and limiting leakage, it is necessary that the pressure drop across this hydrostatic seal, called .DELTA.p, should exceed a certain minimum.
In the case of the primary pumps used at the present time, this pressure limit is of the order of 14 bars.
When the nuclear reactor is operating normally, the reactor cooling water is at a pressure of the order of 150 bars and the cold water injected upstream of the hydrostatic seal is at a slightly higher pressure, so that the pressure drop across the hydrostatic seal is very high, usually close to 150 bars. Satisfactory functioning of the hydrostatic seal is then ensured.
This is no longer the case when the pressure of the primary circuit falls, e.g., when the reactor is stopped, because the cold water injection pressure must be reduced when the primary circuit pressure falls. The flows injected into the volute and into the seal must in fact be balanced, and these flows depend on the pressure of the primary circuit.
Below a certain value of the pressure in the primary circuit, the injection pressure upstream of the hydrostatic seal is no longer sufficient to ensure a .DELTA.p higher than 14 bars, and the hydrostatic seal can no longer function correctly.
In the case of the primary pumps used in the pressurized water nuclear reactors in service at the present time, it is considered that the minimum primary fluid pressure below which the hydrostatic seal can no longer be operated is of the order of 26 bars.
On the stoppage of a pressurized water nuclear reactor, it is necessary to leave at least one primary pump in operation in order to permit the circulation of the primary fluid and to ensure good cooling.
At the end of the cooling, the water is at a pressure of 26 bars and a temperature of 70.degree. C. At this temperature it is no longer possible to maintain the pressure of 26 bars by utilizing the liquid/vapor equilibrium in the pressurizer of the reactor, and it becomes necessary to use loading pumps of an auxiliary circuit in order to maintain the pressure.